suppress unused typedef warning

Summary:
We're seeing the following compiler warnings:
```
unused type alias 'char_type' [-Werror,-Wunused-local-typedef]
```
and this breaks compilation since we build with `-Werror`.

I haven't fully understood why & when this warning is triggered (it doesn't always trigger), but by adding a `[[maybe_unused]]` we can suppress the warning.

Reviewed By: vitaut

Differential Revision: D38387281

fbshipit-source-id: a12c9791fcf3eb6bae72a78310ed461f882a9eb7

deps/fmt/include/fmt/format.h

@@ -2360,18 +2360,18 @@ FMT_CONSTEXPR void handle_dynamic_spec(int& value,
   }
 }
 
-#define FMT_STRING_IMPL(s, base, explicit)                                 \
-  [] {                                                                     \
-    /* Use the hidden visibility as a workaround for a GCC bug (#1973). */ \
-    /* Use a macro-like name to avoid shadowing warnings. */               \
-    struct FMT_GCC_VISIBILITY_HIDDEN FMT_COMPILE_STRING : base {           \
-      using char_type = fmt::remove_cvref_t<decltype(s[0])>;               \
-      FMT_MAYBE_UNUSED FMT_CONSTEXPR explicit                              \
-      operator fmt::basic_string_view<char_type>() const {                 \
-        return fmt::detail_exported::compile_string_to_view<char_type>(s); \
-      }                                                                    \
-    };                                                                     \
-    return FMT_COMPILE_STRING();                                           \
+#define FMT_STRING_IMPL(s, base, explicit)                                    \
+  [] {                                                                        \
+    /* Use the hidden visibility as a workaround for a GCC bug (#1973). */    \
+    /* Use a macro-like name to avoid shadowing warnings. */                  \
+    struct FMT_GCC_VISIBILITY_HIDDEN FMT_COMPILE_STRING : base {              \
+      using char_type FMT_MAYBE_UNUSED = fmt::remove_cvref_t<decltype(s[0])>; \
+      FMT_MAYBE_UNUSED FMT_CONSTEXPR explicit                                 \
+      operator fmt::basic_string_view<char_type>() const {                    \
+        return fmt::detail_exported::compile_string_to_view<char_type>(s);    \
+      }                                                                       \
+    };                                                                        \
+    return FMT_COMPILE_STRING();                                              \
   }()
 
 /**